There are thought to be about 600 million people living close enough to active volcanoes to be affected when they erupt. Casualties from volcanic eruptions have been modest (around 300,000 in the last 200 years) compared to other natural hazards, but economic losses and societal disruption can be considerable. New approaches to volcanic hazard assessment and risk management are emerging as Society requires more information to respond to volcanic emergencies. Extreme eruptions perturb global climate for several years and can have severe environmental impacts and consequences. The modern globalised world is arguably uniquely vulnerable to very large volcanic events, making the study of their return periods, possible environmental effects and consequences a key goal of volcanology.